Well started brightly in the opening stages and in the first ten minutes created a couple of decent openings.
David Clarkson saw a shot saved by Mark Howard, which was followed by a Marc Fitzpatrick header from a Steven Hammell free-kick which sailed just over the bar.
But it was the Saints who took the lead with the first real attack on 12 minutes. Billy Mehmet's through ball sent Andy Dorman clear and despite his first effort being blocked by Graeme Smith, he had all the time in the world to blast the rebound high into the roof of the net.
The horrendous Fir Park pitch then took over as both sides struggled to cope with the strange bounces and bobbles the ball was taking.
The only other points of note in the first period were two needless yellow cards for the visitors with Mehmet booked for rolling the ball away and Jim Hamilton cautioned for sarcastically applauding the referee.
At the start of the second period Motherwell introduced loan signing Cillian Sheridan, but this did little to change the standard of play. This was highlighted by a Mehmet shot which was sliced 25 yards wide of goal.
Motherwell were virtually camped in the Buddies half without ever looking threatening until they scored the equaliser on 66 minutes.
A Clarkson cross from the right picked out Sheridan, whose effort was blocked by Howard. The ball fell kindly for Stephen Hughes to fire home his first goal of the season from 12 yards out.
St Mirren almost won the tie in stoppage time when Scott Cuthbert got a toe to a Steven Robb free-kick, but Smith reacted well to save low to his left.
In the end both sides were delighted to get a second bite of the cherry in this cup tie and a replay that will hopefully be played on grass.